There are number of electrocatalysts for fuel cells are known in the art, such as Platinum and other precious metals based electrocatalysts have proven to be effective in catalyzing the rate of ORR (Oxygen reduction reaction) in Polymer electrolyte membrane fuel cells (PEMFC). However on account of their high cost, scarcity, and less durability in the electrochemical environment, their commercialization is affected. Although Pt and transition metal alloy based electrocatalysts are best known ORR catalysts (in PEMFC), the major lapse lies in the metal corrosion as well as electrochemical stability associated with them. Henceforth finding an alternate so as to dramatically enhance the ORR, along with reducing the higher cost apart from rendering electrochemical stability than conventional catalyst is the need of the hour. In that direction increasing the mass transfer and creating more active sites are the two ways to improve the kinetics of the ORR.
The research on developing metal-free electrocatalysts for oxygen reduction reaction (ORR) has gained substantial attention for economic purpose, where graphene-based carbon nitride nanosheets as efficient metal-free electrocatalysts for oxygen reduction reactions is reported in Angewandte Chemie International Edition 50, (23), pp 5339-5343, 0.2011 by Dr. Shubin Yang. Further the semiconducting properties and emerging applications of polymeric carbon nitrides in photocatalysis and photoelectrochemical energy conversion is reported by Zhang et al. in Science of Advanced Materials, 4, (2), 2012, pp. 282-291. Also preparation of graphitic carbon nitride (g-C3N4)/WO3 composites is reported by Katsumata K et al. in J Hazard Mater. 2013; 260: 475-82.
The doping is found to alter the electron density of carbon, thereby, generating more active sites, needed for ORR. In recent times metal free N-doped carbon materials are getting more attention due to their potential in enhancing the kinetics of the ORR, high electrochemical stability, CO tolerance and cost effective. Thus these materials can act as efficient substitutes for Pt to reduce the cost, and thus promote the commercialization of fuel cell technology.
Template based carbon synthesis has recently gained momentum wherein, a suitable carbon precursor viz., chitosan, glucose, etc. along with a nitrogen source is being used for deriving N doped carbons with high surface area, and good mechanical stability. However, the template removal often involves harsh acid treatment thereby causing undesirable changes in the carbon properties during post-treatment.
On account of their self-sacrificial nature, high carbon content and inherent porosity, these coordination polymers have been recently used to derive highly porous carbon structures thereby ruling out the need for any additional carbon sources. They could be directly used for anchoring the incoming hetero atoms. Among the N precursors, by virtue of its high N content, graphitic carbon nitride (g-C3N4) has previously proven to be an effective catalyst for many reactions such as photocatalytic hydrogen generation water splitting such as Xinchen Wang et al. disclosed an abundant material, polymeric carbon nitride that can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor in Nature Materials 8, 76-80 (2009).
Article titled “Nanoporous Graphitic-C3N4@Carbon Metal-Free electrocatalysts for Highly Efficient Oxygen Reduction” by Y Zheng published in J. Am. Chem. Soc., 2011, 133. (50), pp 20116-20119 reports that a g-C3N4@carbon metal-free oxygen reduction reaction (ORR) electrocatalyst and synthesized by uniform incorporation of g-C3N4 into a mesoporous carbon to enhance the electron transfer efficiency of g-C3N4. The resulting g-C3N4@carbon composite exhibited competitive catalytic activity (11.3 mA cm−2 kinetic-limiting current density at −0.6 V) and superior methanol tolerance compared to a commercial Pt/C catalyst. Furthermore, it demonstrated significantly higher catalytic efficiency (nearly 100% of four-electron ORR process selectivity) than a Pt/C catalyst. The proposed synthesis route is facile and low-cost, providing a feasible method for the development of highly efficient electrocatalysts.
Article titled “Graphene-Based Carbon Nitride Nanosheets as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reactions” by S Yang published in Angewandte Chemie International Edition, May 27, 2011, Volume 50, Issue 23, pages 5339-5343 reports Sandwich-like, graphene-based carbon nitride nanosheets (G-CN), among many other advantages, show an enhanced electrical conductivity. Oxygen atoms can thus access the catalyst surface easily and the rapid diffusion of electrons in the electrode during the oxygen reduction process is facilitated. G-CN nanosheets can hence serve as metal-free electrocatalysts for oxygen reduction reactions (ORR) with excellent performance.
Article titled “Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells” by L Qu published in ACS Nano, 2010, 4 (3), pp 1321-1326 reports the synthesis of nitrogen-doped graphene (N-graphene) by chemical vapor deposition of methane in the presence of ammonia. The resultant N-graphene was demonstrated to act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction via a four-electron pathway in alkaline fuel cells. To the best of our knowledge, this is the first report on the use of graphene and its derivatives as metal-free catalysts for oxygen reduction. The important role of N-doping to oxygen reduction reaction (ORR) can be applied to various carbon materials for the development of other metal-free efficient ORR catalysts for fuel cell applications, even new catalytic materials for applications beyond fuel cells.
Article titled “From metal-organic framework to nanoporous carbon: toward a very high surface area and hydrogen uptake” by H L Jiang published in J Am Chem Soc. 2011 Aug. 10; 133(31):11854-11857 reports a zeolite-type metal-organic framework as both a precursor and a template and furfuryl alcohol as a second precursor, nanoporous carbon material has been prepared with an unexpectedly high surface area (3405 m2/g, BET method) and considerable hydrogen storage capacity (2.77 wt % at 77 K and 1 atm) as well as good electrochemical properties as an electrode material for electric double layer capacitors. The pore structure and surface area of the resultant carbon materials can be tuned simply by changing the calcination temperature.
CN103178273A discloses electrocatalyst of MOFs (Metal-organic Frameworks)/graphene composite carrier and a preparation method of the electrocatalyst. The electrocatalyst is composed of the MOFs/graphene composite carrier and an active component. The preparation method of the MOFs/graphene composite carrier comprises the following steps of: preparing MOFs/graphene composite carrier through a hydrothermal reaction method; and loading the active component on the composite carrier by adopting a microwave synthesis method.
However, the pristine g-C3N4 material is found to exhibit limited activity due to its low electrical conductivity and minimum surface area. Therefore, one of the ways to overcome this limitation could be the incorporation of g-C3N4 into a highly mesoporous and conductive carbon backbone thereby creating a much better and closer requisite for ORR.